1. Field of the Invention
The subject invention generally pertains to dock levelers and more specifically to the structure around a hinge that pivotally connects a lip extension plate to the deck of a dock leveler.
2. Description of Related Art
A typical truck loading dock of a building includes an exterior doorway with an elevated platform for loading and unloading vehicles such as trucks and trailers. Many loading docks have a dock leveler to compensate for a height difference that may exist between the loading dock platform and an adjacent bed of a truck or trailer. A typical dock leveler includes a deck that is hinged along its back edge so that the deck can pivotally adjust the height of its front edge to an elevation that generally matches the height of the rear edge of the truck bed.
In addition, usually a front hinge pivotally connects an extension plate or lip to the front edge of the deck. The front hinge allows the lip to pivot between a stored, pendant position and an extended, operative position. In the extended position, the lip can rest upon the truck bed to form a bridge between the deck and the bed. This allows personnel and material handling equipment, such as a forklift truck, to readily move on and off the vehicle during loading and unloading operations.
The design of the front hinge and the adjoining structure determines some key features of the dock leveler. These features include the crown, openness of the front hinge, flexibility of the deck, and part count.
First, the crown is the angle or apex created between the upper surfaces of the deck and an extended lip. The crown angle is usually about two or three degrees less than 180 degrees, and it becomes an important feature when the truck bed is higher than the platform from which the deck extends. When the truck bed is higher than the platform, the deck must lie at an incline to elevate the deck's front edge. If the lip were coplanar with the deck, whereby no crown existed, the lip would project its distal edge (tip of the lip) a little above the truck bed. This would create a detrimental step between the tip of the lip and the truck bed, wherein the step might trip dockworkers or jar forklifts. A crown eliminates this step by pointing the tip of the lip back down against the truck bed.
It can be difficult, however, to maintain a desired crown as the weight of a forklift repeatedly carrying loads over the front hinge area of the dock leveler generates tremendous forces that tend to flatten the crown over time. Numerous hinge and lip structures have been devised to counter such forces; however, many of them rely on a single abutment subjected to high stress concentrations. A common example of such an abutment would be the back edge of the lip abutting the front edge of the upper deck plate. With such a design, high stress concentrations can eventually wear down the abutment to diminish or eliminate the crown. Other dock levelers, such as disclosed in U.S. Pat. No. 5,815,871, involve assemblies having intricate cross-sectional shapes that can be expensive to produce.
The second mentioned feature of the dock leveler pertains to the openness of the front hinge, which connects the lip to the deck. The openness refers to any gaps that may exist at the hinge when the lip is at its pendant position. U.S. Pat. No. 3,587,126 provides an example of a dock leveler whose front hinge is of an open design, while U.S. Pat. No. 6,370,719 shows a hinge of a more closed design. Although either style may function well, some believe that a closed-style hinge looks cleaner, provides less of a pinching hazard, and prevents items from accidentally falling between the pendant lip and the deck.
Third, torsional flexibility of the deck or the ability of the top surface of the deck to twist out of its normal plane of flatness is another important feature of a dock leveler. Such flexibility allows the deck to twist so that the front edge of the deck and the lip extending from it can align themselves to the plane of the truck bed. For instance, if the dock platform is level but the truck bed lies at an angle from the right to left side of the truck, the deck can twist accordingly.
The deck's torsional flexibility is partially determined by the existence or absence of a front header. When present, the front header usually lies just behind the front hinge and extends generally perpendicular to a series of beams that support the upper plate of the deck. These beams are typically welded or other wise affixed to the header—which thus serves to structurally tie the beams together. Often, the front hinge is mounted directly to the front header. U.S. Pat. No. 6,370,719 shows an example of a dock leveler with a front header, and U.S. Pat. No. 3,587,126 shows one without a header. Some decks with headers are torsionally too stiff, while other decks without headers can be too flexible. Without a header, the top plate of the deck may be so flexible and unsupported that the weight of forklifts may deform the plate in the areas between its supporting beams, whereby the plate becomes permanently dished down in those areas.
Fourthly, the part count of a front hinge assembly can have a direct affect on the cost of a dock leveler. Generally, the number of parts is preferably kept to a minimum, which can be difficult to do when the hinge assembly comprises a series of spaced-apart hinge segments that are pivotally connected by a hinge pin.
Consequently, a need exists for a front header and hinge assembly that provides a dock leveler with a minimal number of parts, a long lasting crown, a generally closed hinge, and a deck with appropriate flexibility.